Anodizing magnesium alloys



Patented Dec. 21, 1948 ANODIZING BIAGNESIUM ALLOYS Herbert K. De Long, Midland, Mich, assignor to The Dow Chemical Company, Midland, ieh, a corporation of Delaware No Drawing. Application December 26, 19%, Serial No. 718,590

3 Claims.

The invention relates to an improved method of coating articles of magnesium-base alloys by anodic oxidation. It more particularly concerns an improvement in the method of anodically coating articles of magnesium set forth in my patent application Serial No. 560,037, filed October 23, 1944.

According to the aforesaid patent application, the articles to be coated are subjected to anodic oxidation in a solution consisting essentially of a strong base dissolved in a mixture of water and at least one water-soluble alcohol or alcoholether, the solution being maintained at a temperature between about 50 and about 150 (3., a temperature of 70 to 80 C. being most advantageous. In general, the water-soluble alcohol component of the solution may be a monohydric alcohol, such as methanol or ethanol, a polyhydric alcohol, such as ethylene glycol, propylene glycol, glycerol, or mannitol, or an alcohol-ether, such as diethylene glycol or mixtures of two or more of these substances. Those alcohols and alcohol-others containing at least two hydroxyl groups are preferred because of their lower volatility which minimizes loss by evaporation. The alcohol or alcohol-ethers are customarily added to the treating solution in a concentration between about 50 and about 500 milliliters per liter of solution, values of 100 to 200 milliliters being particularly suitable. As the base, the alkali metal hydroxides, such as sodium, potassium, and lithium hydroxides may be used in a concentration from at least 30 grams per liter up to its solubility limit, concentrations of 90 to 120 grams being especially advantageous. In anodizing an article in the foregoing aqueous alkaline alcoholic solution, they are subjected to electrolysis with either A. C. or D. C. at a sufficient potential to produce a current density at the article between about 1 and 100 amp-eres per square foot and preferably about to 20 amperes. A low potential, usually 3 to 10 volts is preferred. Coatings are obtained in 1 to 60 minutes electrolysis with optimum results in about to 30 minutes.

In the practical application of the foregoing method of anodic oxidation on a wide variety of articles of magnesium-base alloys, I have found that certain difficulties arise from time to time. One of these is that the coated articles are not always uniform in appearance although protection from corrosion afforded by the coating is uni form. Another disadvantage is that upon rinsing the coated articles after their removal from the anodizing bath, following completion of anodization, portions of the coatings occasionally spall or flake off, leaving some areas denuded of protective coating.

It is the principal object of the present invention to provide an improved sequence of steps in the anodizing process of my said patent application whereby the difiiculties of obtaining uniformity of appearance and the tendency for the coating to flake off or spall during rinsing are overcome. Other objects and advantages will become apparent as the description of the invention proceeds.

The invention is predicated upon the discovery that by soaking in the anodizing bath the articles to be treated both before and after the anodizing operation for a period of about 2 to 10 minutes so that the anodizing solution is in contact with the articles before and after anodizing for at least 2 minutes, the coatings obtained are uniform in appearance and do not spall or flake off when rinsed following removal from the anodizing bath. The invention, then, consists of the method hereinafter fully described and particularly pointed out in the claims.

In carrying out the invention, the articles to be anodized are prepared for the treatment in the usual way as by wire brushing or sanding, is

necessary, for the removal of foreign matter from the surface, and by washing in an appropriate cleaning liquid to remove grease and the like or by pickling as understood in the art. After the articles to be anodized are suitably cleaned, they are subjected to the first operation in the sequence constituting the invention. This operation consists in subjecting the articles to the action of the anodizing solution, as by soaking therein, but without applying a potential to the articles, for a period of about 2 to 10 minutes, the anodizing solution having the composition of my said patent application as set forth above. During this immersion or soaking operation, the surface of the articles undergoes slight, if any, visual change yet the soaking produces a marked effect in the nature of the final coating obtained.

After the articles have undergone an initial soaking in the anodizing bath for the stated period, the anodizing potential is applied to the articles which are not removed from the solution once immersed therein until the conclusion of the treatment in the anodizing solution. The anodizing step already described with reference to my said patent application is used.

Following the anodizaticn and before removal from the anodizing solution, the anodized articles are left to soak in the anodizing solution for at Example Test panels of Dowmetal FS-l (a magnesiumbase alloy having a nominal composition of 3 per cent aluminum, 0.3 per cent manganese, 1 per cent zinc, the balance being magnesium) 3" x 6 x 0.04" were subjected to a conventional pickling operation (e. g., the conventional chromepickle) in preparation for treatment according to the method of the invention. Five lots of the pickled test panels were soaked in the anodizing bath by immersion therein as the first step. The

anodizing bath had the following composition:

caustic soda 2&0 g. p. 1., ethylene glycol 70 ml. per liter, the balance being water. After 5minutes soaking-in the bath, the anodizing potential was appliedusingabout 4 to 6 volts D. C. and producing a current density at the panels of about amperes per square foot. Anodizing was continued for minutes whereupon the application of the anodizing potential was discontinued and each lot of the anodized panels allowed to soak in the bath for a different interval of time before Withdrawing, rinsing, and drying with the results tabulated below:

'iest Results Lot Duration ofSoaking After Anodizing Blank (no soakin 0:5 minute l 1.0 minute 2.0 minutes Coating partly 'spallcd off. Coating chipped off badly on edges. Slight chipping. of coating on edges. Uniform appcaring'coatiug. No chipping, spelling, or flaking of coating 5 .0 .minutes In each of the foregoing tests, anodization was preceded by an initial 5 minute soaking in the anodizing bath. When this initial soaking was omitted, the coatings finally obtained were not uniform in appearance.

I claim:

1. In a method of producing an anodic coating upon an article of a magnesium-base alloy wherein the article is subjected. to electrolysis at a current density between 1 and 100 amperes per square foot and a potential not exceeding about 10 volts in a solution consisting of water, an alkali metal hydroxide in a concentration of at least 30 grams per liter of solution, and a water-soluble aliphatic hydroxyl compound selected from the group consisting of methanol, ethanol, ethylene glycol, proplyene glycol, glycerol, mannitol, diethylene glycol in a concentration between and 500 milliliters per liter of solution, said solution being maintained at a temperature between and C., the steps which consist in soaking the article in the solution both before and after the electrolysis for :a period of about 2 to 10 minutes, the article remaining immersed in the solution from the beginning to the end of the method.

2. The method according to claim 1 in which the alkali metal hydroxide is lithium hydroxide, and the water-soluble aliphatic hydrox-yl compound is ethylene glycol.

.3. The method according to claim 1 in which the alkali metal hydroxide is sodium hydroxide, and the water-soluble aliphatic hydroxyl compound is ethylene glycol.

HERBERT K. DE LONG.

REFERENCES ()ITED The following references are of record in the file of thispatent:

UNITED STATES BATENTS Number Name Date 2,250,473 DeLong July 29, 1941 FOREIGN PATENTS Number Country Date 696, 601 Germany Sept. 25, 1940 

